Which neural network is appropriate for measuring object dimensions from stereo images?
Is the image taken from a constant distance?

If yes, you'd need to scale the images to the same dimensions first of all. For few images say 100-500 images (more the better) you'd need to label the dataset by proper scaling.

Once labeled, use it to train a CNN (Although best would be training a ResNet). Once trained with decent accuracy, test it for the rest of your dataset.

I did something similar for one of my projects, check it out if you want to here.